is it a screw that you need a screwdriver for or is it a screw with a he head that you use a socket or wrench on?
Most likely what happened is that you have a steel screw that threads into an aluminum housing. Aluminum and steel are elecrochemically dissimilar becasue the aluminum is a more active metal than the steel so when the 2 are in contact with eachother some galvanic action takes place. What happens is the aluminum gives up electrons out of its outter shell to the steel, in other words it corrodes and the aluminum bonds to the steel.
There's a few things that you can try to reduce the strength of this bond. You can shock it, If you use a screwdrive to turn the screw insert the proper screwdriver into the head of the screw, if it uses a socket then use a punch against the head. Hit the end of the screwdriver or the punch with a hammer a bumch of times. The shock load will help break up the corrosion and allow the screw to come out easier.
You can try repeatedly applying penetratin oil to the screw for a while. Say during the weekend when you're not planning to drive the car, say friday after work wait till the engine cools down and spray the screw with some penetrating oil. Do this fairly often, say do it again before you go to bed, then again when you wake up saturday morning. Then try doing the shocking thing I just mentioned and try taking the screw out. If the head of the screw is so badly stripped you can't get a screwdriver to grab on then in a last ditch effort try using a pair of vise frips on teh head of the screw. When you clamp the vise grips on make sure that you have them adjusted so that you have to squeeze them as hard to lock the pliers this way they can get a good bite out of the screw and see if you can back the screw out that way.
another method is the flame wrench method. What you do is heat the screw and its sourrounding structure till it gets real hot with a torch. You have to be real careful this way for several reasons. It is possible to heat the aluminum engine block enough to damage it. You can also start a fire especially if there are wires or fluid lines for power steering, brakes or fuel near by. You can also catch the firewall insulation or the hood insulation on fire too.
Then there is the very last ditch effort known as extraction. What you do here is get a drill and an EZ-OUT. Usually the EZ-OUT will say what size drill to use. What you do is drill a hole straight into the screw. You want to drill dead center in the middle of the screw head. Drill deep enough so that the EZ-OUT can go in far enought to get a good bite. Then using a tool to turn the EZ-OUT turn it in the direction to loosen the screw. Be careful, don't put too much pressure on the EZ-OUT they are made of very hard metal and are brittle and can snap off in the head of screw making life miserable. If the EZ-OUT doesn't work then the next course is to use a drill that is the same size of the screw shank and you drill into the head until it comes off. Then you can remove the part and deal with the screw with the part off the engine. Now you should have some of the prodruding screw shank to grab onto. Again try the soaking with penetrating oil thing again and shocking with a hammer. Firm taps work good, don't whail on it or something might get damaged. Then try taking the rest of the screw out with vise grips again. If that doesn't work either than you have to go all the way and drill the bolt all the way out. This is where some skill is needed to keep the drill centered on teh screw. What you want to do is use a drill that is the same size as the screw again and drill the screw all the way out, threads and all. Once thats done you need to figure out if the screw was metric or standard and what the thread pitch is. I'm sure that the part is held on with more than just one screw so use another screw to compare to. What you'll want to do now is get a HELI-COIL kit to repair the threads.
The HELI-COIL kit comes with a special sized drill bit, a special thread tap, an instalation tool and a few inserts. What you do is follow the directions on the container. basically use the supplied drill to enlarge the hole a little, then use the tap to thread the hole and use the instalation tool to thread in an insert. When you get the insert in all the way you break off the instalation tang and cut off the extra coil that sticks out of the hole and you now have brand new threads inside the hole. You just now need to pick up a new screw.
This time when you put the part back together you can make sure that you use some anti-sieze compound on the threads of the screw to help make sure that this never happens again.
